In recent years, the number of patients suffering from diseases of respiratory organs such as asthma, pulmonary emphysema, chronic bronchitis and the like has tended to increase and oxygen inhalation therapy has been available as one of most effective therapies for them. Such oxygen inhalation therapy is a treatment method for patients to inhale oxygen-enriched air. As the supply source of the oxygen and the air, an oxygen concentrator, a liquefied oxygen cylinder, an oxygen gas cylinder and the like are known but an oxygen concentrator has become dominant for home oxygen therapy because of convenience and easiness in maintenance and control.
An oxygen concentrator is an apparatus in which about 21% oxygen present in air is concentrated in a high concentration to supply to a user. Such apparatuses include a membrane type oxygen concentrator using a polymer membrane, which selectively permeates oxygen, a pressure swing adsorption-type oxygen concentrator using an adsorbent, which selectively adsorbs nitrogen relative to oxygen, an apparatus electrochemically separating oxygen contained in air to supply and the like. The pressure swing adsorption-type oxygen concentrator is mainly used in a market because oxygen concentrated to 90% or more can be obtained in good yield.
The pressure swing adsorption-type oxygen concentrator is an apparatus using an adsorption bed filled with molecular sieve zeolites such as 5A type zeolite, 13X type zeolite, Li-X type zeolite, MD-X type zeolite and the like as the adsorbent, which can selectively adsorb nitrogen relative to oxygen, supplying compressed air from a compressor to the adsorption bed to adsorb nitrogen under a pressurized condition and isolating unadsorbed oxygen as a concentrated-oxygen gas. Such an apparatus is generally equipped with two or more adsorption beds and can continuously produce oxygen by switching between the absorption step in which nitrogen is adsorbed by the adsorbent in the adsorption beds to produce unadsorbed oxygen and the desorption step in which nitrogen is regenerated by evacuating absorbed nitrogen by decompressing the other absorption beds.
The pressure swing adsorption method includes a PSA method, in which the pressure of the desorption step is reduced to an atmospheric pressure and a VPSA (vacuum pressure swing adsorption) method, in which an inner pressure of an adsorption cylinder is reduced to a vacuum level using a compressor in order to increase the regeneration rate of the adsorbent and both methods are used as an oxygen concentration method in a medical oxygen concentrator.
Such an oxygen concentrator is often used not only at hospital, but also at home as oxygen inhalation therapy for patients with chronic respiratory diseases. Accordingly, not only stable and consistent supply of a predetermined concentration of an oxygen-concentrated gas at a constant feed rate is required for the oxygen concentrator as main performance, but also requirement for the secondary performance such as low power consumption, quietness, miniaturization and the like are increasing taking into consideration the convenience of a user to the apparatus.
Among constitutional parts of the oxygen concentrator, a compressor consumes most of electric power used in the whole apparatus and at the same time, occupies a certain space within the enclosure as well as generates a major source of noise. A compressor used has to be miniaturized in order to realize such secondary performance of the apparatus. However, miniaturization of the compressor is accompanied by the reduction of a flow rate and compression performance in the compressor, resulting in disadvantage with reduced allowance to maintain adsorption-desorption performance of the oxygen concentrator.
As a means to achieve lowering power consumption of an oxygen concentrator and realize a high yield of oxygen, Japanese Patent Application Laid-Open No. H11-207128 describes an apparatus, in which the number of rotation of a compressor is controlled in response to a predetermined flow rate of oxygen to control a supply flow rate of raw material air and a switching timing between an adsorption step and a desorption step is determined by controlling opening and closing of a switching valve to supply air in an optimum pressurized time.
Japanese Patent Application Laid-Open No. 2001-259341 also describes an apparatus provided with a function, in which the number of rotation of a compressor is controlled by use of an inverter based on an oxygen concentration of an oxygen-concentrated gas produced and a predetermined flow rate of oxygen, and the power consumption is suppressed at a low flow rate.
As an apparatus to shorten a pressure rising time of an adsorption cylinder in the adsorption step and to improve the adsorption efficiency in an early stage, Japanese Patent Application Laid-Open No. H6-31129 discloses a gas separation apparatus, in which the number of rotation of the motor driving the compressor is increased for a predetermined period at the adsorption step, while otherwise the number of rotation is controlled in a normal value.